Plasma Arc Machining - A Comprehensive Guide

Plasma arc machining (PAM) is a non-traditional machining process that uses a high velocity plasma jet to melt and vaporize workpiece material. This process is capable of accurately machining a wide range of conductive materials. In this comprehensive guide, we will cover all aspects of plasma arc machining including its working principle, process parameters, applications, advantages, limitations and more.

Introduction to Plasma Arc Machining (70 words)

Plasma arc machining is a thermal material removal process that uses a constricted arc formed between a tungsten cathode and the workpiece which serves as the anode. A high velocity, high temperature plasma jet is produced that melts and vaporizes the workpiece material, resulting in material removal. PAM can accurately machine high strength, difficult-to-machine conductive materials.

Working Principle of Plasma Arc Machining

The plasma arc machining process works on the principle of thermal energy produced by a plasma jet causing melting and vaporization of the workpiece material. The key components used in the process are:

• Plasma torch - It consists of a tungsten cathode and a copper nozzle anode. Plasma gas (nitrogen, hydrogen or argon) flows between the cathode and anode. A DC arc is generated between the electrodes that ionizes the gas to form a plasma jet capable of reaching temperatures up to 28,000°C.
• Power supply - A DC power supply generates the required high current (100-1000 amps) low voltage (40-100 volts) arc between the electrodes to produce the plasma.
• Plasma gas - Gas like nitrogen, argon or hydrogen is used to generate the plasma. Nitrogen is commonly used.
• Cooling system - Water cooling is provided for the torch electrodes and worktable to control thermal expansion.
• Workpiece - The workpiece serves as the anode in the electrical circuit. It is mounted on the worktable.

The plasma jet melts and vaporizes the workpiece material as the torch traverses over the work surface. The molten material is removed by the plasma jet flow and forms solid slag. This results in material being removed and a cavity being generated in the workpiece. The process can be used for cutting, drilling and surface modification of metals.

Process Parameters of Plasma Arc Machining

The various process parameters that affect plasma arc machining are:

• Current - Current in the range of 100-1000 amps is used. Higher current increases material removal rate but reduces accuracy.
• Plasma gas flowrate - Gas flowrate affects the plasma jet energy and machining rate. Typical flowrate is 1-5 L/min. Higher flowrate improves machining rate.
• Arc voltage - Voltage of 40-100 V is used. Higher voltage increases energy of plasma jet.
• Torch to work distance - A small torch to work distance of around 1-5mm is maintained. Smaller the distance, higher is the energy density and machining rate.
• Traverse speed - Influence MRR and accuracy. Lower traverse speed improves accuracy.
• Electrode angle - Angle of electrode influences shape and direction of plasma jet. Perpendicular orientation maximizes MRR.
• Electrode ID - Smaller ID electrodes improve jet concentration and accuracy. Larger IDs improve MRR.
• Nozzle size - Nozzle size affects plasma jet diameter. Smaller nozzles improve precision.

By selecting the right combination of the above parameters, the plasma arc machining process can be optimized for maximum material removal rate or improved accuracy.

Applications of Plasma Arc Machining

Plasma arc machining finds applications in machining of the following types of products:

• Cutting and drilling of superalloys like titanium, Hastelloy, Inconel etc. These are difficult-to-machine materials, but can be accurately cut and drilled using PAM.
• Cutting and drilling of tungsten, molybdenum, tantalum and other refractory metals. PAM is one of the few processes capable of machining these metals.
• Making small holes in glass, ceramics, and carbide materials. The plasma jet melts the material allowing hole formation.
• Micromachining applications producing microholes and fine features in hard and brittle materials.
• Cutting of non-metallic materials like plastics, wood and rubber. The plasma jet heat severs the material.
• Removal of failed coating from components and cleaning metal surfaces. The coating material is vaporized by the plasma jet.
• Plasma arc cutting (PAC) uses PAM principles for shape cutting of thick section metals.
• Modified PAM is used for plasma coating deposition and surface modification.

Thus plasma arc machining is ideally suited for accurate machining of all types of conductors that are difficult-to-cut by traditional machining. The process is especially useful for reflective metals like aluminum, copper and noble metals.

Plasma Arc Machining vs Laser Beam Machining

Plasma arc machining and laser beam machining have some similarities, but also important differences:

• Heat source - LASER uses coherent light energy while PAM uses high temperature plasma jet.
• Process range - LASER can cut thicker sections up to 25mm while PAM cuts only up to 75mm thickness.
• Accuracy and surface finish - LASER provides superior accuracy and surface finish compared to PAM.
• Equipment cost - LASER beam machines are more expensive than PAM equipment.
• Process gases - LASER uses inert gases while PAM uses nitrogen, argon etc.
• Generated forces - LASER has negligible cutting forces while significant forces are present in PAM.
• Material versatility - PAM can only cut conductors while LASER can cut all materials including ceramics, composites etc.
• Heat affected zone - LASER has a lower HAZ compared to PAM which can thermally damage workpiece.

While both processes have their pros and cons, laser beam machining produces superior quality cuts compared to plasma arc machining. But PAM equipment cost is lower than laser which makes it more economical for some applications.

What Kind of Products Are Made Through Plasma Arc Machining?

The plasma arc machining process is versatile and can be used to manufacture a variety of products across several industries. Some examples include:

• Cutting turbine blades, compressor blades, engine disks and other components made of nickel, titanium and cobalt alloys for aerospace and gas turbine industry.
• Drilling small holes in automotive fuel injectors and valves made of stainless steel and alloys.
• Cutting heat sinks, electronic packaging and other products made of aluminum, copper alloys for electronics industry.
• Drilling holes in glass ampoules used by pharmaceutical companies.
• Cutting ceramic substrates used in semiconductors and electrical circuits.
• Cutting mining tools and equipment made of tungsten carbide material.
• Drilling holes in nuclear reactor components made of zirconium, titanium and other alloys.
• Cutting parts for medical implants like stents, orthopedic devices made of stainless steel, cobalt, titanium etc.
• Cutting graphite stock for manufacturing graphene, carbon brushes and seals.

Thus, plasma arc machining allows machining products across a diverse range of conductive and non-conductive materials. PAM is especially suited for difficult-to-machine materials not easily cut by conventional methods.

Plasma Arc Machining Equipment

The main equipment used in the plasma arc machining process are:

1. Power Supply

A DC power supply capable of generating output between 100 to 1000 amps at an open circuit voltage of 200 to 400 volts DC is required. Constant current (CC) power supplies are preferred to obtain stable cutting current.

2. Plasma Torch

It consists of a tungsten cathode and copper anode nozzle. Cooling channels circulate water to cool the torch. Different torch styles like transferred arc, non-transferred arc and microplasma jets are available.

3. Plasma Gas Supply

Plasma gas like nitrogen, argon or hydrogen is supplied from high pressure cylinders equipped with regulators and flowmeters. An inert gas shield may also surround the jet.

4. Water Supply

Water supply is needed for cooling the torch electrodes, worktable and workpiece. Deionized water gives better cooling performance.

5. CNC Control

Computer numerical control allows generating complex profiles by synchronizing plasma torch motion with machining parameters.

6. Worktable

A movable worktable with fixtures allows mounting the workpiece. It must be water cooled and be able to resist high heat.

7. Exhaust System

Fumes and gases generated must be removed through an exhaust system. Dust collection system collects removed particles.

8. Safety Systems

Safety interlocks, gas alarms, fire suppression systems are required. Personal protective equipment for the operator is also necessary.

Plasma Arc Machining Process Capabilities

Plasma arc machining is capable of:

• Material removal rate up to 16-20 cubic inches per hour
• Cutting virtually all conductive metals up to a thickness of 75 mm
• Drilling holes of minimum 0.25 mm diameter with aspect ratio up to 40:1
• Achieving accuracy up to 0.025mm and surface finish up to 0.4 microns Ra
• Low taper angle of 1-2 degrees for deep holes
• Curved and complex profile cutting
• Cuts hard, brittle or tough metals like titanium, tungsten, inconel etc.
• No mechanical stresses or burrs induced in workpiece

Thus PAM can machine materials and forms not feasible by conventional machining methods. The high energy density plasma jet makes possible micromachining with good accuracy and surface finish.

Plasma Arc Machining - FAQ

Q1. What is the metal removal mechanism in plasma arc machining?

Plasma arc machining removes metal by melting and vaporizing the workpiece surface through the heat energy of the high temperature plasma jet. The jet temperature reaches up to 28,000°C which is above the melting point of all commercial metals.

Q2. What are the components of a plasma arc machining system?

The main components are power supply, plasma torch, plasma gas supply, cooling system, CNC controls, exhaust system and worktable. The plasma torch consists of a tungsten cathode and copper anode nozzle.

Q3. Why is a non-transferred arc used in plasma arc machining?

In a non-transferred arc, the arc takes place between the electrodes inside the torch. This concentrated plasma jet is focused on the workpiece. This makes drilling small deep holes possible.

Q4. Why is a copper nozzle used in plasma arc machining?

Copper has very high thermal conductivity which allows the heat of the plasma arc to be dissipated effectively by water cooling. This protects the nozzle from rapid erosion by the arc.

Q5. What are the advantages of PAM over conventional machining processes?

PAM can machine very hard metals not possible by conventional machining. It has higher material removal rate and lower tool pressures. No stresses are induced and no burrs are present.

Q6. What are the limitations of plasma arc machining?

The equipment is complex and expensive. Only electrically conductive materials can be machined. The process induces a lot of heat which can thermally damage the workpiece.

Q7. How does a transferred plasma arc machining system work?

In transferred arc PAM, the workpiece serves as the anode of the electrical circuit. The arc takes place between the cathode and workpiece. This allows thicker and faster cutting but with reduced accuracy.

Q8. Why is argon used sometimes as the plasma gas?

Argon provides more energy concentration in the plasma jet allowing higher temperature up to 35,000°C. This gives faster cutting capability with improved accuracy.

Q9. What safety precautions are required in plasma arc machining?

Safety glasses, face shields, gloves and respiratory protection must be used. Proper ventilation, fire suppression system and electrical safety devices are required to prevent hazards.

Q10. How is surface roughness and accuracy achieved in PAM process?

Careful control of process parameters like current, plasma gas flow, cutting speed and torch standoff distance allows achieving good surface finish and dimensional accuracy in plasma arc machining.

Plasma Arc Machining Process

Steps Involved

The sequence of steps involved in plasma arc machining process are:

1. The workpiece is fixed on the worktable and the plasma torch positioned above it.
2. Plasma gas is allowed to flow between the tungsten cathode and copper nozzle anode.
3. A high voltage arc is initiated between the electrodes which ionizes the gas into plasma.
4. The torch is brought close to the workpiece to form an arc that melts and vaporizes work material.
5. The torch is traversed along the desired path to cut or drill holes in the workpiece.
6. The molten material is flushed away by the plasma jet and removed through exhaust.
7. Cooling is provided throughout to the torch, worktable and workpiece.
8. Parameter monitoring and closed loop control is maintained for optimal cutting conditions.
9. Finally, the finished workpiece is removed and cleaned to remove any remaining slag material.

Process Characteristics

• Temperatures up to 28,000°C are generated in the plasma arc. This allows machining of very hard, refractory metals.
• Material removal up to 16-20 cubic inches per hour is possible with depths up to 75mm.
• Holes of minimum 0.25 mm diameter can be drilled with aspect ratio up to 40:1.
• Taper angle as low as 1-2 degrees is achievable for deep holes.
• No mechanical cutting forces are generated minimizing stresses and defects.
• The workpiece surface may undergo some thermal damage and metallurgical changes.
• High capital cost is involved butrunning costs are low as no cutting tools are used.

Methods of Cutting

• Perpendicular cutting - Torch held perpendicular to work surface for high MRR
• Angled cutting - Torch tilted to produce angled cuts
• Shallow angle cutting - Minimize material melt-through for thin sheets
• Circle cutting - Torch traverses in a circle for hole generation
• Straight cutting - Linear torch paths for shape or slot cutting
• Contour cutting - Complex paths for profile cutting

Methods of Drilling

• Perpendicular drilling - Standard drilling of vertical holes
• Angle drilling - Holes produced on an angle other than 90 degrees
• Orbital drilling - Torch rotates around the hole for improved finish
• Microhole drilling - For drilling small, precision holes
• Deep hole drilling - For depth to diameter ratio up to 40:1
• Novel methods like vibration assisted PAM drilling for improved hole quality are also being used.

Industrial Applications of Plasma Arc Machining

Plasma arc machining finds application across several major industries including:

• Aerospace - Cutting titanium and nickel alloys for turbine blades, engine disks, fuselage skin. Drilling cooling holes in turbine blades.
• Automotive - Cutting steel and alloy engine valves. Drilling fuel injector nozzles. Cutting transmission gears.
• Electronics - Cutting semiconductor wafers. Drilling holes in circuit boards and heat sinks. Cutting ceramic substrates.
• Medical - Cutting cobalt-chromium alloys for implants. Cutting titanium prosthetics and devices like stents.
• Tool making - Machining tungsten carbide for cutting tools, dies and gauges.
• Nuclear - Cutting nuclear reactor components made of zirconium, tantalum, titanium.
• Oil and gas - Cutting high strength steels for downhole tools and wellhead equipment.
• Lab research - Cutting ceramics, composites, glass for research samples.

PAM allows micromachining of materials like stainless steel, copper, Kovar, Invar, kovar, tungsten, titanium that are used widely across industries. The aerospace industry in particular uses PAM extensively for hard-to-cut superalloys.

Tungsten Electrodes Used in Plasma Arc Machining

Tungsten is commonly used as the cathode electrode in the plasma arc machining process due to several favorable properties:

• Tungsten has an extremely high melting temperature of 3422°C which allows it to withstand the heat of the plasma arc.
• It offers good electrical conductivity and thermal conductivity for plasma generation.
• Tungsten electrodes have good resistance to erosion caused by the high energy plasma beam.
• They maintain their hardness and strength adequately at the elevated temperatures during PAM.
• Tungsten can be sharpened to a fine tip which helps concentrate the arc on a small work area.
• The coefficients of expansion of tungsten and copper (for nozzle) are similar. This allows their good bonding.
• Tungsten is easily available at moderate cost. Fabricating tungsten electrodes is easy.
• Oxides of tungsten have favorable properties that protect the cathode against oxidation.

To summarize, the unique material properties like high temperature strength, thermal properties, and erosion resistance make tungsten the preferred choice for plasma arc machining electrodes.

Accuracy and Surface Finish of Plasma Arc Machining

Plasma arc machining can achieve good dimensional accuracy and surface finish by selecting suitable process parameters. The typical values that can be attained are:

Accuracy

• Holes can be drilled to accuracy of up to 0.025 mm.
• Linear cutting edges can be made with accuracy up to 0.075mm.
• Curved contours and complex shapes can be cut to accuracy around 0.125mm.

Surface Roughness

• Surface roughness Ra value up to 0.5 microns (0.02 mil) is possible.
• For some fine micromachining applications, surface finish up to 0.1 microns is attainable.
• Curved surfaces tend to have higher roughness than flat surfaces.
• With special techniques like orbital motion of torch, finishing cuts and optimized parameters.

Also, read:

• Electrochemical Machining: Key Features, Factors, Benefits & Applications
• Electron Beam Machining: Working Principle, Key Features & Applications
• Abrasive Jet Machining (AJM): Working Principle, Process Parameter, & Applications
• Ultrasonic Machining - Working Principle, Advantages, Disadvantages, and Applications
• Waterjet Cutting: Working Principle, Equipments, Benefits, & Applications
• Laser Beam Machining: Working Principle, Types, Advantages, Applications & Key Process
• Magnetic Abrasive Finishing (Working Principle, Process Parameters & Applications)